As promised some observations on the state of my GV2000's rad after a 1½ hour motorway run. When I stopped I switched off and lifted the bonnet straight away.
The top hose was too hot to kep my hand in contact with it for any length of time. The bottom hose was cool, surprisingly so. The rad itself had quite a temperature gradient. The top ¼ of it was hot, but in the centre it was definately cool, a bit warmer at the edges.
They say it's never to late to learn, and I must admit I didn't realise that the aircon heat exchanger rad was in front of the engine rad.
And yes indeedy, the fan in front of them is activated by the aircon. So that's why it works in the warmer weather! I use it a couple of times a month during autumn & winter to keep the seals up to scratch. I switch it on at the end of a journey as I park and jumped to the wrong conclusion, i.e. that it was an additional rad cooling fan switching on as a result of slower air flow. D'oh!
Grand Vitara. The most fun you can have with your clothes on.